In order to increase the combustion stability during low speed operation under a small load and to ensure the output during high speed operation under a large load, some internal combustion engines mounted on a vehicle or the like are provided with a variable valve activation system. The variable valve activation system changes lift timing and lift amount of an engine valve, like an intake or exhaust valve. The engine valve is driven to open/close by a drive cam provided to a camshaft of a valve train corresponding to the operation status of the internal combustion engine.
JP 2000-38910A teaches a variable valve activation system for internal combustion engines, which comprises a first arm, which is driven to swing by a drive cam, provided to a camshaft, a swing cam, which is linked with the first arm being interposed by a link mechanism and is driven to swing by the first arm to thereby cause an engine valve to open/close, and a variable mechanism which varies the swing position of the swing cam with respect to the engine valves. In this variable valve activation system, the first arm is provided with a control arm arranged as an eccentric shaft, which is pivoted eccentrically on a swing shaft of the first arm. By the rotation of the eccentric swing shaft, the swing position of the first arm is changed by means of the control arm and the open/close timing of the engine valve and the lift amount are continuously varied.
FIGS. 6(A) and 6(B) illustrate a conventional variable valve activation system 101 for internal combustion engines. The variable valve activation system 101 comprises a first arm 106, which is pivoted on a swing shaft 105 so as to be driven to swing by a drive cam 103 on a camshaft 102 that contacts a roller 104 mounted on the first arm 106. A swing cam 111 is linked with the first arm 106 by a rod 108 of a link mechanism 107 and is pivoted on the camshaft 102 so as to be driven by the first arm 106 to swing to open/close an engine valve 110 being interposed by a tappet 109. A variable mechanism 112, which varies the swing position of the swing cam 111 with respect to the engine valve 110, includes a control arm 113 on which the first arm 106 is eccentrically pivoted on the swing shaft 105 of the first arm 106, so that the swing shaft 105 is arranged as an eccentric shaft.
The variable valve activation system 101 for internal combustion engines is arranged so that, utilizing the rotation of the swing shaft 105 as the eccentric shaft, the swing fulcrum P of the first arm 106 is shifted by the control arm 113 to thereby continuously vary the lift amount of the engine valve 110. Defining the length of the first arm 106 as L1, and the length of the rod 108 as L3, the lift amount of the engine valve 110 results in length L4.
FIGS. 7(A) and 7(B) illustrate another variable valve activation system 201 for internal combustion engines. In FIGS. 7(A) and 7(B), parts in the variable valve activation system 201 having the same function as those in the variable valve activation system 101 shown in FIGS. 6(A) and 6(B) are given with the same second and third reference numerals after numeral “2”. In order to increase the lift amount of the engine valve 210, the length of the first arm 206 is elongated to L1+α, and the length of the rod 208 is elongated to L3+β. As a result, the lift amount of the engine valve 210 is increased to L4+γ. The lift amount of the engine valve 210 can be also increased by increasing the lift amount of the drive cam 203.
However, when the length of the first arm 206 and the length of the rod 208 are elongated, the lift amount of the engine valve 210 can be increased just a little. On the other hand, the space that is necessary for allowing the movement of the first arm 206 and the rod 208, increases greatly. As a result, the variable valve activation system is restricted to be mounted on the cylinder head. Also, the lift amount may be increased by increasing the size of the drive cam 203. However, in order to prevent interference between the maximum lift path of the drive cam 203 and the swing shaft 205, since the distance L5 between the camshaft 202 and the swing shaft 205 (refer to FIGS. 7(A) and 7(B)) has to be increased, the downsizing of the variable valve activation system 201 is adversely restricted.
Further, in the link mechanism 107 of the variable valve activation system 101 shown in FIGS. 6(A) and 6(B), for example, in the case where the swing shaft 105 of the first arm 106 is the eccentric shaft, since the lift amount of the engine valve 110 increases, eccentric amount L6 for the control arm 113 has to be increased. As a result, the size of the link structure within a cylinder head increases, resulting in an increase of the restrictions on the head structure (for example, interference between the swing shaft and the drive cam or the like).
Furthermore, for example, in the variable valve activation system 101 shown in FIGS. 6(A) and 6(B), compared to an ordinary rotating cam (for example, drive cam 103), the operating angle of the swing cam 111 as the final cam that acts on the valve lift becomes smaller. Due to the cam profile, there may arise a problem such that lubrication becomes poor (increase of friction and wear), or optimization of the valve lift may be restricted (for example, increase of external diameter of the tappet).
It is an object of the invention to provide a variable valve activation system for internal combustion engines capable of continuously varying lift amount of the engine valve. Particularly, the variable valve activation system is capable of downsizing of the variable valve activation system without reducing lift amount of the engine valve to thereby increase the mounting performance and provide a compact arrangement for the variable activation system as mounted on an internal combustion engine.